Troubleshooting Small Engines
Objectives
• The Student Will Be Able To:
• List, summarize, and employ the 5
troubleshooting rules;
• Interpret common symptoms of problems that
occur in small engines; and
• Compile the information given in this laboratory
section in order to diagnose and correct small
engine problems throughout the remainder of
the semester.
Troubleshooting
•
The skills involved in
troubleshooting engine problems
are very similar to the skills used
by medical doctor in diagnosing
health problems. As a
troubleshooting "doctor",
students must know the causes
of certain symptoms that your
mechanical patient might have.
Luckily mechanical medical
school or a residency is not
required before attempting to
work on a patient. All that is
needed are the 5 simple rules to
troubleshooting small engines.
• The five troubleshooting rules should be
followed in sequence in order that the maximum
amount of time and money be saved. These five
rules are also very general; this allows them to
work for just about all makes and models of
small engines.
Remember these 5 Rules for
troubleshooting engine problems
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Spark
Compression
Accessories
Carburetion
Timing
Rule #1: Spark
• Spark refers to the high voltage spark that is created
by the ignition system. Without spark the engine is
completely unable to function. Checking the ignition
for "Spark" is the first rule because of its simplicity.
•
To check to see if the ignition system is
functioning properly a spark tester is used. As
mentioned is Laboratory II there are numerous types
and ways to test for "Spark", and they are all
effective. If the ignition system is creating an
adequate "Spark", but still does not start, continue
on to Rule #2. If the ignition system fails to create a
"Spark", check this list of items for possible
solutions.
• http://www.youtube.com/watch?v=qAVz6ICOkKU
Possible “Spark” Solutions
• Courtesy of Briggs & Stratton Corp.
▫ 1. Check the spark plug, it may be faulty.
▫ 2. Check the armature air gap. (Figure 6.0)
▫ 3. Check to see if the flywheel key is
sheared. (Figure, 6.1 and 6.2)
▫ 4. Check for an incorrect breaker point
gap.
▫ 5. Check for dirty or burned breaker
points.
▫ 6. Check for a worn or stuck breaker
plunger.
▫ 7. Check for a shorted ground wire or stop
switch.
▫ 8. Check for condenser failure.
▫ 9. Check for armature failure.
▫ 10. Check for worn bearings and/or shaft
on the flywheel side.
Fig. 6.1, Flywheel Keys
Fig. 6.2, Sheared Flywheel Key
Fig. 6.0, Armature
& Flywheel
Magnetron® Ignition Systems:
• Starting in 1982, all Briggs & Stratton engines were
equipped with Magnetron® ignition armatures, leaving
Breaker Points and Condensers by the wayside. The
Magnetron® ignitions are much more reliable, but if you
still fail to get a “Spark” you might check:
• 1. To see if the Stop Switch is grounded.
• 2. The interlocking system, if equipped.
• 3. Armature air gap too wide.
• 4. Ignition armature ground wire eyelet disconnected.
• 5. Broken external wires at the ignition armature.
• 6. Blown ignition armature from 12 volt battery current.
Rule #2: Compression
• A great deal of compression is needed in the cylinder
in order that the air-fuel mixture ignites properly. An
engine with little compression may drastically lose
power or may fail to start at all. Compression is the
second rule because it is almost as quick and simple
as checking for Spark.
•
Check for Compression in one of two ways. The
first method involves removing the spark plug and
inserting a compression gauge in its place, in order to
measure how many psi the piston exerts on the
cylinder head. With the gauge threaded into the
cylinder head the starter rope should be pulled two to
three times. If the engine has a starter, the engine
should be turned over for about two to three seconds.
Either of these should give a fairly accurate reading
on the gauge. The second method does not require
you to remove the spark plug. All that a person needs
to do to determine compression using the second
method is to spin the flywheel clockwise. If the
flywheel rebounds sharply the engine is considered to
have adequate compression for ignition.
•
If the engine has adequate compression, but still
fails to start, move on to Rule #3. If the engine does
not have adequate compression the following list of
check points should be considered.
If the engine does not have
adequate compression the following
list of check points should be
considered.
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Check For:
1. A loose spark plug
2. Loose cylinder head bolts
3. A blown head gasket
4. Burnt valves and/or seats
5. Insufficient tappet clearance
6. A warped cylinder head
7. Warped valve stems
8. Worn or scared cylinder walls
9. Worn or broken piston rings
10. Broken connecting rod
Rule #3: Accessories
• Accessories are all of those
mechanisms that control, or are
controlled by the engine. Many
times faulty, broken, or jammed
accessories such as belts, pulleys,
electrical equipment, blades,
controls, etc.. may be the cause of
the engine’s failure to start. A
thorough check of all accessory
equipment can solve many
starting problems.
•
If the engine has no accessory
problems, but still fails to start,
continue on to Rule #4. If a
problem is found, it should be
corrected and periodically
checked for reoccurrences.
Rule #4: Carburetion
• The carburetor is a complex part of the small
engine that might have several reasons for
causing starting difficulty.
•
Before disassembling the carburetor,
check for the obvious! Check the fuel line,
the fuel filter, the fuel tank, and the fuel
itself for water or contaminants. Any of
these may restrict ignition. If the fuel
contains water, drain all liquid from the tank
and disassemble the carburetor’s fuel pump
or bowl and clean; now reassemble the
carburetor and fill the tank with fresh fuel.
Many times a quick check of these items will
solve the problem and save you work.
•
Basic carburetor adjustments are given
below. The needle valve seats are made of
soft brass and can be easily damaged. When
turning the needle valve in, take care not to
use force because you may damage the
needle valve and the seat, (See Figure, 6.3).
Loosening the packing nut will allow the
screw to turn with greater ease. If fuel seeps
from around the adjustment screw, tighten
the packing nut.
BASIC CARBURETOR
ADJUSTMENT:
•
Gently turn the
needle valve until it
just closes. The valve
may be damaged if it is
closed tightly. Next,
open the needle valve
1-1/2 turns counterclockwise. The initial
adjustment will permit
the engine to be
started. Now warm-up
the engine (approx. 5
minutes) in
preparation for final
adjustment.
FINAL ADJUSTMENT:
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Place the throttle control in the “fast” position. Turn the needle
valve clockwise until the engine just starts to slow. Now open
the needle valve 3/8 of a turn counterclockwise. Then rotate
the throttle arm counterclockwise and hold it against the
throttle stop while adjusting the idle RPM by turning the idle
speed adjusting screw to obtain 1750 RPM. Release the throttle
and the engine should accelerate smoothly. If it doesn’t, the
carburetor should be re-adjusted, usually to a slightly richer
mixture by turning the needle valve counterclockwise 1/8 turn
more. (See Figure, 6.4).
Fig. 6.4
The most common carburetor problems related to
starting can be corrected by disassembling, cleaning,
adjustment, and replacing old gaskets/diaphragms in the
carburetor. This is a “shotgun approach”, cure-all for correcting
carburetor problems. Other carburetor problems may be
caused by:
1. Over choking.
2. Excessively rich fuel mixture.
3. Valve malfunction.
4. Faulty floats or springs.
5. Leaking gaskets.
6. Broken or clogged inlet tubes.
7. Broken needle valve assembly.
8. Fuel tank on/off valve, turned “off”.
If you have completed work on the carburetor and the engine
still fails to start, move to Rule #5!
Rule #5: Timing
• Checking and resetting the
timing is the most time
consuming troubleshooting step
because it involves draining the
engine oil and removing the
crankcase cover, therefore, it is
done last.
•
If the timing is off, the
engine will be out of
synchronization, and will not
start. The timing can be
corrected by draining the oil,
removing the crankcase cover,
and realigning the timing dots
that are marked on the
crankshaft gear and the cam
gear. (See Figure, 6.5 and 6.6).
Finally, reassemble the engine
and refill the crankcase with oil.
Fig. 6.5, Aligning Timing Marks
Fig. 6.6, Aligning Timing Marks
• *Note: One of these Five Rules should
correct the engine’s starting problem.
Many times the engine will start, but
problems persist. Such problems
may cause poor or erratic engine
performance. The following are some
small engine trouble symptoms and
their probable causes.
Vibration:
• Rotating parts out of balance.
• Bent crankshaft.
• Loose mounting bolts.
• Carburetor not properly adjusted.
• Mounting deck cracked.
Runs Rough/Won’t Accelerate Properly:
• Incorrect spark plug gap.
• Engine not warm.
• Low fuel level.
• Idle set too low.
• Fuel mixture too lean.
Backfires While Throttling Up or Down:
• Water in fuel.
• Low fuel level.
• Fuel mixture too lean/rich.
• Choke malfunction.
• Dirty carburetor.
• Kill switch contacting.
• Leaking/worn valves.
Dies at High Idle:
• Low fuel level.
• Low oil level.
• Loose spark plug lead.
• Carburetor out of adjustment.
Will Not Reach High Idle RPM:
• Choke not disengaged completely.
• Throttle control loose or installed improperly.
• Clogged fuel filter.
• Fuel line too small.
• Low fuel level.
• Fuel tank too far from engine.
• Fuel pump malfunction.
• Gravity feed tank below carburetor.
• Trash in carburetor.
• Carburetor out of adjustment.
• Incorrect spark plug gap.
Starts & Dies Quickly:
• Worn, torn, and/or faulty carburetor diaphragm.
• Low fuel level.
• Obstruction in fuel line or carburetor.
• Carburetor needs to be disassembled and cleaned.